A three-dimensional display using holography is well known as a display for producing a three-dimensional image in a space. The holography is a technique of recording and reproducing a three-dimensional image by recording both an amplitude and a phase of a light wave on a plane. A three-dimensional image thus produced can be seen at different angles from different viewing points. In comparison with other methods, this method can produce natural three-dimensional image that can satisfy all physiological factors (i.e., parallax, convergence, eye accommodation and so on) of human eyes when recognizing a stereoscopic subject.
A hologram to be used therein is prepared by recording a pattern produced by interference occurred between light scattered from an object (signal beam) and light from the other direction (reference beam). When the hologram is illuminated by light of the same wavelength as the reference light, it acts as a, diffraction grating and reproduces the same wave fronts as the original signal light, thus forming an object image in a space.
The holographic display usually represents a still picture, but several methods have been proposed to represent holographic moving pictures. One of the methods is the real-time video-holography display which was developed by Professor Benton et. al. of the MIT Media Laboratory.
In the real-time video-holography display, a hologram signal of a three-dimensional image is computed and inputted into an acousto-optic modulator (AOM). The laser light modulated by the AOM is scanned vertically by a galvanic mirror and horizontally by a polygon mirror.
Although the AOM can not simultaneously display all holograms necessary for reproducing a three-dimensional image, it can display holograms corresponding to a whole image within a specified duration by vertical and horizontal scans.
The horizontal scanning by the polygon mirror also compensates a flow of interference fringes, thus acting upon an image to be still. As a diffraction angle of light from the AOM is about 3 (degrees), the hologram is reduced in size to form an image through a reducing optical lens system, thus obtaining a visible area 5 to 6 times as large as the diffraction angle.
The use of the above-mentioned scanning and reducing optical systems can produce a virtual hologram about an output lens, which can be seen as a three-dimensional image through the output lens and a diffusing plate. The diffusing plate diffuses the light in a vertical direction to widen the visible area.
A monochromatic display system is as described above. A color display system can be realized by using three AOMs which correspond to three color lasers, respectively, a helium-neon (He--Ne) red laser of 632.8 nm, a neodymium-dopted (Nd) YAG green laser of 532.0 nm and a helium-cadmium (He--Cd) blue laser of 441.6 nm.
Japanese Laid-Open Patent Publication No. 6-186901 discloses a three-dimensional image display device which is simple in construction and capable of producing a holographic three-dimensional image.
In the display device, a laser beam emitted from a laser is enlarged through an enlarging lens and then falls as a reference beam onto a specified area on a hologram disc which can be rotated by a motor.
In this instance, the hologram disk has a plurality of holograms disposed in a circle thereon and bonded thereto. Each hologram is prepared so as to reproduce a virtual image of a point light source at a different position in a three-dimensional space.
Consequently, as the hologram disk is rotated from the motor, the holograms can be illuminated one by one by a reference laser beam, subsequently reproducing a virtual image of a point light source at plural different points in a three-dimensional space.
When holograms are formed so that virtual images of a point light source may be densely reproduced as, e.g., rectangles arranged in rows and columns of a matrix, each being disposed at respective different positions in a three-dimensional space, a desired image according to image signals can be reproduced by suitably selecting and displaying virtual images of these point light sources in the matrix. A rectangular box is floating in a space can be seen when all virtual images are lightened.
Japanese Laid-Open Patent Publication No. 6-82612 discloses a three-dimensional image display device using a diffraction grating array. In a diffraction grating array which is a plane substrate whereon a plurality of cells consisting of diffraction gratings is arranged. The cells are divided into areas corresponding to pixels of respective parallax images to represent a three-dimensional image. This method is featured by producing an image having a parallax.
In this conventional device, a diffraction grating array is prepared by arranging a plurality of cells, each consisting of a plurality of pixels on a flat substrate. Each cell is spatially divided by the pixels into areas with near slope and distance of gratings and the areas of the pixels correspond to respective parallax images. The diffraction grating array is used as a basic device capable of displaying a three-dimensional image having a parallax.
Another well known conventional three-dimensional image display with a diffraction grating array comprises a diffraction grating array, liquid crystal display elements, which are spatial light-modulating elements disposed behind the diffraction grating array, and color filters disposed behind the liquid crystal display elements. If a micro-area is observed in this three-dimensional image display, a color filter layer selects light of a certain wavelength from white incident light and a liquid crystal element selects transmission or shut-off of light. The transmitted light reaches a diffraction grating array.
The diffraction grating array made of light-transmitting resin plate or the like material diffracts light passing therethrough. The outgoing direction of the diffracted light is decided by the diffraction angle of the light which depends upon the gradient and the intergrating space of the microarea. This microarea is bright with the selected wavelength light when being observed from the diffraction angle direction of the light. A three-dimensional image can be produced by controlling each of microareas according to the total image to be produced.
Another conventional three-dimensional image display for reproducing a stereo-image in a space is based upon a volume scanning method. This type three-dimensional image display is featured in that it does not require an observer to use special glasses and can produce a three-dimensional image to be naturally focused.
The volume scanning type three-dimensional image display system is composed basically of a laser light source, a modulator, an X-Y deflector, a control computer, an image data memory and a screen panel.
The operation of the display is as follows:
Data on a three-dimensional image to be displayed is first prepared and the screen panel moves at a constant speed over the image starting from an initial display position to an end display position and instantly returns to the initial position, then repeats the same movement. Sections of a three-dimensional image, which correspond respective positions of the screen panel, are scanned by raster scanning with laser beams through the modulator and the X-Y deflector under the control of the control computer. The rasters of laser light are subsequently projected on the screen panel. A three-dimensional image is seen by the after image of vision in a space defined by the area.times.the movement stroke of the screen panel if the movements of the screen panel and the laser light are sufficiently rapid and synchronized with each other.
A real image type three-dimensional image display is described in Japanese Laid-Open Patent Publication No. 56-500313. The principle of this prior art device is as follows:
Light-emitting elements such as LED are two-dimensionally disposed to form a plane with a light-emitting element array which rotates about an axis being within the plane to produce a three-dimensional video by the after image of vision.
A three-dimensional image display using diffraction gratings is disclosed in Japanese Laid-Open Patent Publication No. 4-311916. This prior art device is based upon the method of deflecting light beams crosswise relative to incident light by using a diffraction grating which is prepared by disposing dot-like diffraction patterns on a plain substrate.
A liquid-crystal spatial light-modulator attached tight to the diffraction grating panel substrate is driven from a liquid crystal drive to rearrange a right parallax image and a left parallax image into an image which can be recognized as three-dimensional video according to the principle of stereogram.
The above-mentioned real-time video holography display, however, requires considerable time to compute interference fringes of a three-dimensional image, a large capacity data memory for storing a large amount of calculation results and high-speed data transmission. It is, therefore, much difficult to display a large-size three-dimensional image.
The three-dimensional image display using holograms involves such problems that a large size hologram must be prepared for representing a large size three-dimensional image and be driven by a large-sized driving mechanism and, furthermore, a three-dimensional image to be observed by one person can be prepared with processed hidden lines but a three-dimensional image to be observed by a number of persons is in principle semi-transparent to its rear side. The three-dimensional image display using an array of diffraction gratings can represent three-dimensional image only using a hologram.
The volumetric scanning type display can only provide a semi-transparent three-dimensional image to be seen to the rear side similarly to the case of the hologram type display.
The prior art device described in Japanese Laid-Open Patent Publication No. 56-500313 can not process hidden line and hidden dots in principle and reproduces an image with hidden lines and dots, which is limited to use.
The prior art device described in Japanese Laid-Open Patent Publication No.4-311916 is based upon stereogram system requires a viewer to look two parallax images by left eye and right eye respectively. This may tire the eyes of the viewer when viewing it for a long time.
As described above, the conventional three-dimensional display devices are difficult to represent a large-size stereo-image and may tire the eyes of viewers in particular when viewing stereogram type images.
The semi-transparent images in which the rear side also appears can be used for very limited applications such as three-dimensional representation of a CT image having previously cut fragments, air-traffic control radar display for indicating spatial locations of aeroplanes and the like.